The maximum usable frequency of an ionospheric layer at 60° incidence and with 8 MHz critical frequency is

A. Waveguide match B. Cavity match C. Direct coax-helix match D. All of the above

A. A long time constant B. Low-pass filters C. The shortest possible time D. The restricted high-frequency response

A. $${\text{S}} = \frac{{2\lambda {\text{R}}}}{{\text{L}}}$$ B. $${\text{S}} = \frac{{3\lambda {\text{R}}}}{{\text{L}}}$$ C. $${\text{S}} = \frac{{\lambda {\text{R}}}}{{\text{L}}}$$ D. $${\text{S}} = \frac{{\text{L}}}{{\lambda {\text{R}}}}$$

A. To focus the beam of primary electrons B. To shield the electron beam from unidirectional magnetic C. As a second anode and to prevent the build-up of secondary field D. All of these